It'll fit, Kirk, but by the time you clear the loading port, you may have a problem with eye relief. That 13.5" clearance isn't a hard and fast rule, it's just based on my experience with a CFX and various scopes. Lots of variables, scope eye relief, your own preference in scope setup, LOP of the rifle and how long your arms are, all of these will impact how long a scope can be.
BTW, I think a 6-18 is probably more scope than you need for a CFX, I always felt 3 or 4-12 was about perfect for most springers, unless you shoot FT or BR. Good luck.
Dave